Control communications in communications networks

ABSTRACT

A communications network has data and dedicated control network communication links between nodes. The control network is automatically and reliably created by each node determining each of its neighbor nodes to which it has a direct data link, each node having at least two neighbor nodes, and establishing a control network link for communicating control network traffic directly with each of these neighbor nodes. Each control network link can, if a dedicated control network link is not available, use bandwidth of a data link between the respective nodes, a link selection protocol being based on shared risk link groups and available bandwidth.

This invention relates to communications networks, and is particularlyconcerned with providing a control network for reliably communicatingcontrol traffic among nodes in a communications network.

Although the invention is described below in the context of an opticalcommunications network, the term “communications network” is used hereinto embrace any network of nodes among which communications can takeplace via communications links of any form. As used herein, the term“data” refers to user traffic which it is a purpose of thecommunications network to carry; data or user traffic is carried in thecommunications network via communications paths or links which arereferred to as data or in-band links. In contrast, the term “control” isused herein to refer to other, non-user, traffic in the communicationsnetwork which may be provided for control, administration, networkmanagement, or other purposes; control traffic may be carried usingavailable bandwidth of in-band links and/or on other communicationspaths or links which do not carry data or user traffic, and are referredto as control or out-of-band links.

BACKGROUND

It is known to provide a communications network, comprising nodes andcommunications links between the nodes, in which redundancy is providedfor maintaining at least some communications among the nodes in theevent of failures in the network. For example, in the case of an opticalcommunications network in which optical fibers, which may carry WDM(wavelength division multiplexed) optical signals, providecommunications links among optical nodes of the network, protectionswitching of communications against a failure in the network, forexample due to a fiber cut or a node failure, can be provided byre-routing the optical signals via different fibers provided redundantlyfor this purpose.

Whether or not such a communications network includes such protectionswitching for its data or user traffic, it must also be set up andarranged to carry necessary non-user or control traffic, for example foradministration and network management purposes. To this end, it is knownfor a network operator to engineer and manually configure or provisionthe communications network to provide for the necessary control trafficcommunications. As indicated above, the control traffic can usebandwidth of the same communication links as are used by the data oruser traffic, and/or dedicated control traffic paths which are not usedfor data. The control traffic paths can be considered as a controlnetwork which is overlaid on the data or user traffic paths andconstitutes a subset of the communications network.

Manual provisioning of the control network takes time and is prone toerrors, and these factors become increasingly significant withincreasing complexity of the communications network. In addition, in theevent of a fault in the communications network, the control traffic ofsuch a manually provisioned control network may not be protected by anyprotection switching or redundancy that is provided.

Consequently, a need exists for an improved method of providing controlcommunications in communications networks, and consequently improvedcommunications networks.

SUMMARY OF THE INVENTION

According to one aspect of this invention there is provided a method ofautomatically creating a control network comprising communication linksfor communicating control network traffic between nodes of acommunications network, comprising the steps of, in each of a pluralityof nodes:

establishing a list of a plurality of neighbour nodes to each of whichthe node has a direct communication link for communicating user trafficwith the neighbour nodes; and

establishing a control network link for communicating control networktraffic directly with each of said neighbour nodes in said list.

Another aspect of this invention provides, in a communications networkcomprising a plurality of nodes, first communication links forcommunicating user traffic between nodes, each node having at least oneof said first communication links with each of a plurality of respectiveneighbour nodes, and second communication links for communicatingcontrol network traffic between at least some of the nodes, a method ofautomatically creating a control network comprising the steps of, ineach node: determining each of said plurality of respective neighbournodes; and establishing, using one of said second communication links orbandwidth of one of said first communication links, a control networklink for communicating control network traffic directly with each ofsaid respective neighbour nodes.

Preferably the step of establishing a control network link forcommunicating control network traffic directly with each of saidrespective neighbour nodes comprises, in the event of a fault adverselyaffecting a control network link using one of said second communicationlinks, maintaining the control network link using available bandwidth ofone of said first communication links. This ensures, to the extentpermitted by the communications network and the fault, that the controlnetwork is self-healing in the presence of the fault.

The invention also provides a communications network comprising aplurality of nodes, first communication links for communicating usertraffic between nodes, each node having at least one of said firstcommunication links with each of a plurality of respective neighbournodes, and second communication links for communicating control networktraffic between at least some of the nodes, wherein each of the nodes isarranged for carrying out this method.

The invention further provides a communications network comprising aplurality of nodes and a plurality of communication links forcommunicating user traffic and control network traffic between thenodes, wherein each of the nodes is arranged for determining a pluralityof neighbour nodes to each of which the node has a direct communicationlink for communicating user traffic, and is further arranged forautomatically establishing and maintaining a control network link forcommunicating control network traffic directly with each of saidneighbour nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood from the following descriptionby way of example with reference to the accompanying drawings, in which:

FIG. 1 illustrates a simple optical communications network incorporatingan embodiment of the invention;

FIG. 2 is a flow chart indicating steps which can be performed by a nodeof the network of FIG. 1 in carrying out an embodiment of the invention;

FIG. 3 illustrates a communications network arrangement for use indescribing a shared risk link group;

FIG. 4 is a flow chart illustrating a link selection protocol; and

FIG. 5 is a messaging diagram relating to the link selection protocol.

DETAILED DESCRIPTION

Referring to FIG. 1, a simple optical communications network isillustrated as comprising a plurality of, in this example six, nodes 10,identified individually as Node 1 to Node 6, and a network managementstation (NMS) 12, which are coupled together via optical communicationspaths or links represented by solid and dashed lines.

As is known in the art, such a communications network can include anarbitrary number of nodes 10 and network management stations 12, whichmay be located together or separately from one another, with variousarrangements of communications links among the nodes 10 and NMSs 12having any desired configuration or topology. Furthermore, such acommunications network can be coupled to other similar or differentcommunications networks in various manners. Accordingly, FIG. 1 servesmerely to illustrate a simple form of network for the purposes ofdescribing an embodiment of the invention.

The optical communications paths or links represented by solid anddashed lines in FIG. 1 comprise data or in-band links 14, represented bysolid lines and serving primarily for carrying data or user traffic, andcontrol or out-of-band links 16, represented by dashed lines and servingfor carrying control traffic as explained above. The data links 14 andcontrol links 16 are subsets of the overall set of communications linksof the communications network.

Conveniently, and to a large extent as shown in FIG. 1, the data orin-band links 14 and the control or out-of-band links 16 parallel oneanother to provide respective separate communications links for data andcontrol traffic. For example, in FIG. 1 Node 1 has separate paralleldata and control links 14 and 16 respectively to Node 2, and separateparallel data and control links 14 and 16 respectively to Node 5. Eachnode to which a node has a direct data or in-band path is referred to asan adjacent node or a neighbour node; for example Node 2 and Node 5 areadjacent or neighbour nodes for Node 1.

However, such a predominant parallel arrangement of separate data andcontrol links 14 and 16 is not essential. For example, FIG. 1illustrates a control link 16 between Node 2 and Node 4 without anyparallel data link. Furthermore, FIG. 1 illustrates a data link 14between Node 2 and Node 3 without any separate control link betweenthese nodes. In the latter case, in accordance with this embodiment ofthe invention and as further described below, a portion of thecommunications bandwidth on the data link 14 between Node 2 and Node 3is used for control traffic.

Similarly, a portion of the communications bandwidth on any of the dataor in-band links 14 between adjacent nodes 10 can be used for controltraffic in accordance with this embodiment of the invention as furtherdescribed below. For example, such a situation may arise in the eventthat the bandwidth of a control link 16 is insufficient for the controltraffic to be carried by the link, for example in the event of a faulton the control link 16.

In accordance with embodiments of this invention, a requirement formanual provisioning of the control network, and an associated risk oferrors in such manual provisioning, is avoided by implementing in eachnode 10 a control network automatic creation (CNAC) process which(unless disabled as described below) is activated when the node ispowered up and remains in effect while the node is active or inoperation.

This CNAC process allows the nodes 10 of the network to automaticallycreate and maintain the control network with a topology, or subset ofcontrol traffic links, that provides for the control traffic and forredundancy so that, to a reasonable degree, the control network can bemaintained despite failures in the communications network. Thus thecontrol network is automatically created and maintained with a desirablelevel of reliability. If, for example as in the case of Node 6 in FIG. 1as described below, the CNAC process can not be satisfied for aparticular node, an alarm can be provided to alert a network operator tothis situation.

In accordance with this CNAC process, each node is required to have atleast one control network link (i.e. path of the control network forcontrol traffic) to each of its adjacent or neighbour nodes, and to haveat least two adjacent or neighbour nodes.

The CNAC process is desirably also implemented using a particular linkselection protocol, as described below.

FIG. 2 illustrates steps of the CNAC process in the form of a flowchart, steps of which are followed individually by each node 10 of thenetwork. On power up and consequent initialization of a node, and whilethe node is active in the event of a change (e.g. addition, removal, ordeletion of a link due to a failure) of communication links, the CNACprocess is entered at a block 20 in FIG. 2.

As a part of this process block 20, the respective node 10 establishesin known manner a list of its adjacent or neighbour nodes, to each ofwhich it has a direct data or in-band link 14. As the data or in-bandlinks 14 represent a base topology of the communications network, thislist in each node is referred to as a base neighbour list. In a similarmanner, the respective node 10 establishes a list of its adjacent orneighbour nodes to which it also has a direct connection via a controllink 16. This list is referred to as a control network (CN) neighbour(NBR) list.

With respect to the establishment of the base neighbour and controlnetwork neighbour lists, and generally with respect to the operation ofthe communications network, it is noted that the network can operate inaccordance with known operating procedures and protocols, including forexample the so-called OSPF (open shortest path first) link-state routingprotocol in accordance with IETF (Internet Engineering Task Force)document RFC2328. Such procedures and protocols also provide so-calledLSAs (link state advertisements), which can be extended to so-calledopaque LSAs as documented in IETF document RFC2370. Opaque LSAs, whichcan be used by embodiments of this invention as described below, providefor example for communication of propriety messages (i.e. messages notunderstood by the OSPF protocol) between nodes of the communicationsnetwork.

At a subsequent block 21 in FIG. 2, the respective node 10 determineswhether the CNAC process is enabled for this node, and if not (i.e. ifthe CNAC process is disabled for this node) proceeds to a return block22 without further action. This enables the automatic creation of thecontrol network to be disabled for one or more individual nodes, inwhich case manual provisioning of the control network can be carried outfor such nodes in known manner.

If the CNAC process is enabled for this node as determined at the block22 in FIG. 2, then as indicated at a block 23 the node compares itscontrol network neighbour list with its base neighbour list. In asubsequent decision block 24 the node 10 determines whether all of theadjacent nodes in the base neighbour list are also present in thecontrol network neighbour list, i.e. whether the control networkneighbour list completely contains the base neighbour list.

Referring again for example to FIG. 1, it can be appreciated that thiswill be the case for Node 1, because this node has a respective controlnetwork link 16 to each of its adjacent or neighbour nodes, namely Node2 and Node 5. Thus Node 1 will have a base neighbour list identifyingNode 2 and Node 5, and a control network neighbour list also identifyingNode 2 and Node 5.

Similarly, this will be the case for Node 5, which will have a baseneighbour list identifying Node 1 and Node 4, and a control networkneighbour list also identifying Node 1 and Node 4, and also for Node 6,which will have a base neighbour list and a control network neighbourlist each identifying only Node 4. Further, this will also be the casefor Node 4, which as can be seen from FIG. 1 will have a base neighbourlist identifying its adjacent or neighbour nodes, namely Node 3, Node 5,and Node 6, and a control network neighbour list identifying these samenodes (i.e. the base neighbour list is completely contained within thecontrol network neighbour list) and also identifying Node 2 to whichNode 4 has a further control network or out-of-band link 16.

Conversely, this will not be the case for Node 2 and Node 3 as shown inFIG. 1. For Node 2, the base neighbour list will identify Node 1 andNode 3 to which there are direct data links 14, whereas the controlnetwork neighbour list will identify Node 1 and Node 4 to which thereare dedicated control network links 16; consequently, Node 3 in the baseneighbour list is not also identified in the control network neighbourlist. For Node 3, the base neighbour list will identify Node 2 and Node4 to which there are direct data links 14, whereas the control networkneighbour list will identify only Node 4 to which there is a dedicatedcontrol network link 16; consequently, Node 2 in the base neighbour listis not also identified in the control network neighbour list.

Referring again to FIG. 2, if it is determined in the decision block 24that all of the nodes identified in the base neighbour list are alsopresent in the control network neighbour list, then at a decision block25 the node determines whether there are at least two (i.e. more thanone) such neighbour nodes in the base neighbour list. If so, the controlnetwork is determined for the respective node, and the return block 22is reached. In the example of FIG. 1, this would be the case for Node 1,Node 4, and Node 5.

If there are not at least two such neighbour nodes in the base neighbourlist, then at a block 26 an alarm is generated and then the return block22 is reached. In the example of FIG. 1, this would be the case for Node6, which has only one neighbour node. The alarm serves to indicate to anetwork operator that the CNAC process has not met its requirements asdescribed above, and this alarm can be investigated in known manner.

Conversely, if it is determined in the decision block 24 that at leastone node identified in the base neighbour list is not present in thecontrol network neighbour list, then in a decision block 27 the nodedetermines, for example as further described below, whether there is atleast one available in-band, i.e. data or user traffic, link 14 to thisnode with sufficient bandwidth to be used as a control network link. Ifnot, then the block 26 is again reached to generate an alarm indicatinga failure of the automatic process to create the control network inaccordance with its requirements, and again a return is made via theblock 22.

If, however, there is at least one available in-band link withsufficient bandwidth, then a block 28 is reached in which an in-band ordata link is selected (in a manner for example as described below) toprovide the desired control network link, the respective node identifiedin the base neighbour list is consequently added to the control networkneighbour list, and a loop is made back to the decision block 24. TheCNAC process of FIG. 2 thus continues until the control network isautomatically created, in accordance with its requirements as describedabove, for this node, or the automatic creation process fails to meetthe requirements and an alarm is generated accordingly.

As stated above, the CNAC process is activated when each node is poweredup and remains in effect while the node is active or in operation. Inthe latter respect, in the event of a fault in the communicationsnetwork adversely affecting a control network link using, for example,one of the links 16, the resulting link change detected in a node 10results in the node repeating the CNAC process from the block 20 in theflow chart of FIG. 2, thereby maintaining the control network link usingavailable bandwidth of one of the links 14 to the extent that this ispossible within limits imposed by the topology of the communicationsnetwork and the nature of the fault. Thus the control network can beautomatically self-healing within such limits.

It can be appreciated that the CNAC process as described above takesplace individually in each node 10. In the case of the network of FIG.1, which illustrates only one in-band or data link 14 between Node 2 andNode 3 having no dedicated control network link directly between them,Node 2 and Node 3 would each separately determine at the block 27whether this link has the necessary available bandwidth for the controlnetwork link; if so, select this link accordingly; and, if not, generatean alarm as described above.

More generally, however, there may be a plurality of in-band or datalinks between two nodes between which it is desired to automaticallycreate a control network link as described above, and it is desirable toprovide a link selection protocol to facilitate an optimum selection ofthe same link by the two nodes.

In an embodiment of this invention, such a link selection protocol isbased on two attributes which are associated with each of the links,these two attributes being an available bandwidth (ABW) and a sharedrisk link group (SRLG). The ABW is the bandwidth on the respective linkthat is available for reservation, and hence which can (if the bandwidthis sufficient) potentially be reserved to provide a control networklink. The SRLG is an identifier which indicates for each link a sharedrisk with one or more other links which may be used for the controlnetwork. This is explained further below with reference to FIG. 3.

FIG. 3 illustrates a simple communications network arrangement in whichthere are three nodes 10, identified as Nodes A, B, and C, with in-bandor data links between them illustrated by solid lines as in FIG. 1, anddedicated control network or out-of-band links between them representedby dashed lines also as in FIG. 1. As illustrated in FIG. 3, there arefour in-band or data links 14-1 to 14-4 between Node A and Node B, butthere is no dedicated control network or out-of-band link between thesenodes. Conversely, Node C has both an in-band or data link and adedicated control network link to each of Nodes A and B.

From the description of the CNAC process described above, it can beappreciated that this CNAC process, applied to the arrangement of FIG.3, will cause each of Nodes A and B to try to select one of the in-bandlinks 14-1 to 14-4 and to reserve available bandwidth on the selectedlink for use as a control network link between these nodes, in order tomeet the requirements of the CNAC process.

Each of the links 14-1 to 14-4 has attributes including an ABW and aSRLG as discussed above. For the purposes of explanation, it is assumedfor example that the available bandwidth on the links 14-1 to 14-4progressively decreases in this order of these links, as represented bythe respective ABWs of these links. Further, it is assumed that the ABWof the link 14-4 is not sufficient for the control network link desiredbetween Nodes A and B, so that this link is not suitable for selectionfor this purpose and is excluded at the decision block 27 in FIG. 2.

In addition, as illustrated by an ellipse 30 in FIG. 3, it is assumedthat the links 14-3 and 14-4 have a shared risk with the control networklink 16-1 (and also with the in-band link) between Nodes A and C. Forexample, such a shared risk may arise from these links being ondifferent fibers in the same conduit. Consequently, the links 14-3,14-4, and 16-1 have a common SRLG identifier to denote this shared risk.

It can be appreciated that, if either of the in-band links 14-3 or 14-4were to be used to provide the control network link between Nodes A andB to meet the requirements of the CNAC process, then a single fault suchas a cut of the conduit including these links could isolate Node A fromthe rest of the control network. For this reason, the link 14-3 isexcluded, and the link 14-4 is also excluded, from selection by each ofNodes A and B for providing the required control network link betweenthese nodes. This exclusion is based on these in-band links having acommon SRLG identifier with another control network link, in this casethe link 16-1, in the network.

Accordingly, each of Nodes A and B selects one of the in-band links 14-1and 14-2 to provide the required control network link. For loadbalancing purposes, this selection is arranged to choose the in-bandlink having the greatest ABW, in this example the link 14-1 as indicatedabove.

FIG. 4 is a flow chart illustrating the link selection protocol,corresponding to the block 28 in the flow chart of FIG. 2 and reachedafter determining, at the block 27 in FIG. 2, that there is at least onein-band link with sufficient available bandwidth to be used for thedesired control network link.

For clarity in the following description with reference to FIG. 4, thenode in which the steps of FIG. 4 are taking place is referred to as thesource node, and the node to which the respective in-band links extendfrom the source node is referred to as the neighbour node. It can beappreciated that, as each node operates independently, similar steps cantake place with these nodes interchanged.

Referring to FIG. 4, in a block 40 any links having a SRLG identifierwhich is common to any other control network link is eliminated asdescribed above; although not shown in FIG. 4, if this eliminates allpotential in-band links then the CNAC process fails and an alarm isgenerated in the same manner as described above. In a subsequent block41, the source node selects the in-band link to the neighbour nodehaving the greatest available bandwidth. Thus the blocks 40 and 41represent the selection of the link on the basis of the two attributesdiscussed above.

As represented by a block 42, the source node then sends an opaque LSA,as discussed above, to the neighbour node, specifying the amount ofbandwidth that should be reserved for carrying control network trafficon the selected in-band link. If the neighbour node agrees to therequested bandwidth and is not currently negotiating use of a differentlink with the source node, then it responds with an ACK (positiveacknowledgement) opaque LSA. The source node determines this at a block43, and accordingly reaches a block 44 in which it initializes thecontrol network link using the reserved bandwidth on the selectedin-band link, and updates the control network topology (i.e. its controlnetwork neighbour list) accordingly, thereby completing the linkselection process.

If the neighbour node does not agree to the requested bandwidth or(acting itself as the source node) is currently negotiating use of adifferent link with the source node (acting as the neighbour node), thenit can instead respond with a NACK (negative acknowledgement) opaque LSAincluding a corresponding error code, with the source node consequentlyreaching a block 45 in which it responds in accordance with the errorcode.

For example, the neighbour node may indicate that the requestedbandwidth on the selected in-band link is not available, in which casethe source node can mark this link as not available for carrying controlnetwork traffic. Alternatively, the neighbour node may, if it has ahigher IP (Internet Protocol) address than the source node, choose adifferent in-band link for use as the control network link, indicatingthis as part of the opaque LSA data. In this case the source nodeattempts to reserve the necessary bandwidth for the control network linkon the specified in-band link. The condition of a higher IP address inthis case serves to resolve possible contention between the source andneighbour nodes.

This aspect of the link selection protocol is explained further withreference to the messaging diagram of FIG. 5, in which t1 to t8represent times at which opaque LSA messages are sent and received bythe Nodes A and B of FIG. 3 for selection of one of the links 14-1 and14-2 between these nodes. In FIG. 5, vertical lines represent Nodes Aand B, inclined arrowed lines represent messages and their directions,time advances downwardly as indicated, and it is assumed that Node B hasa higher IP address than Node A. The situation represented by FIG. 5could for example occur in the situation described above in relation toFIG. 3 if the links 14-1 and 14-2 have the same available bandwidth.

In FIG. 5, at the time t1 Node A, acting as the source node, sends anopaque LSA message to use the link 14-2 as the control network link withNode B, and this is received by Node B at the time t3. At the time t2(which is illustrated as being between the times t1 and t3, but couldalternatively be before the time t1 or after the time t3) Node B, actingas the source node, sends an opaque LSA message to use the link 14-1 asthe control network link with Node A, and this is received by Node A atthe time t4. On the basis of its higher IP address, in response to themessage it receives to use the link 14-2 Node B sends a NACK at the timet5, and this is received by Node A at the time t7. Conversely, on thebasis of its relatively lower IP address, in response to the message itreceives to use the link 14-1 Node A sends an ACK at the time t6, andthis is received by Node B at the time t8. Thus after the times t7 andt8 both Nodes A and B have agreed to use the link 14-1 as the controlnetwork link between them.

It can be appreciated from the above description that the CNAC processserves automatically to create the control network with a controlnetwork link to each of its adjacent or neighbour nodes, of which theremust be at least two to meet the CNAC process requirements describedabove, so that there is an inherent reliability of the resulting controlnetwork. Furthermore, in the event of a fault in the communicationsnetwork the CNAC process continues to be active in each node to maintainand, if necessary, replace disrupted control network links usingavailable bandwidth of in-band or data links, so that reliability of thecontrol network continues to be maintained.

Although particular embodiments of the invention are described above, itcan be appreciated that numerous modifications, variations, andadaptations may be made without departing from the scope of theinvention as defined in the claims.

1. A method of automatically creating a control network comprisingcommunication links for communicating control network traffic betweennodes of a communications network, comprising the steps of, in each of aplurality of nodes: establishing a list of a plurality of neighbournodes to each of which the node has a direct communication link forcommunicating user traffic with the neighbour nodes; and establishing acontrol network link for communicating control network traffic directlywith each of said neighbour nodes in said list; wherein thecommunication links comprise first links for communicating user trafficbetween nodes and second links for communicating control network trafficbetween nodes, and the step of establishing a control network link forcommunicating control network traffic directly with each of saidneighbour nodes in said list comprises establishing the control networklink using one of said second links where this is available between therespective nodes, and otherwise establishing the control network linkusing available bandwidth of one of said first links between therespective nodes.
 2. A method as claimed in claim 1 wherein the step ofestablishing the control network link using available bandwidth of oneof said first links between the respective nodes comprises selecting oneof a plurality of said first links between the respective nodes having agreatest available bandwidth.
 3. A method as claimed in claim 1 whereineach of said communication links has an identification of shared riskwith others of said communication links, and the step of establishingthe control network link using available bandwidth of one of said firstlinks between the respective nodes comprises excluding any of said firstlinks having a shared risk with any other control network link.
 4. Amethod as claimed in claim 3 wherein the step of establishing thecontrol network link using available bandwidth of one of said firstlinks between the respective nodes comprises selecting one of aplurality of said first links between the respective nodes having agreatest available bandwidth.
 5. A method as claimed in claim 1 whereinthe step of establishing a control network link for communicatingcontrol network traffic directly with each of said neighbour nodes insaid list comprises, in the event of a fault adversely affecting acontrol network link using one of said second links, maintaining thecontrol network link using available bandwidth of one of said firstlinks between the respective nodes.
 6. In a communications networkcomprising a plurality of nodes, first communication links forcommunicating user traffic between nodes, each node having at least oneof said first communication links with each of a plurality of respectiveneighbour nodes, and second communication links for communicatingcontrol network traffic between at least some of the nodes, a method ofautomatically creating a control network comprising the steps of, ineach node: determining each of said plurality of respective neighbournodes; and establishing a control network link for communicating controlnetwork traffic directly with each of said respective neighbour nodes byusing one of said second communication links where this is availablebetween the respective nodes, and otherwise establishing the controlnetwork link by using available bandwidth of one of said firstcommunication links between the respective nodes.
 7. A method as claimedin claim 6 wherein the step of establishing the control network linkusing bandwidth of one of said first communication links comprisesselecting one of a plurality of said first communication links betweenthe respective nodes having a greatest available bandwidth.
 8. Acommunications network comprising a plurality of nodes, firstcommunication links for communicating user traffic between nodes, eachnode having at least one of said first communication links with each ofa plurality of respective neighbour nodes, and second communicationlinks for communicating control network traffic between at least some ofthe nodes, wherein each of the nodes is arranged for carrying out themethod of claim
 7. 9. A method as claimed in claim 6 wherein each ofsaid communication links has an identification of shared risk withothers of said communication links, and the step of establishing thecontrol network link using bandwidth of one of said first communicationlinks comprises excluding any of said first communication links having ashared risk with any other control network link.
 10. A method as claimedin claim 9 wherein the step of establishing the control network linkusing bandwidth of one of said first communication links comprisesselecting one of a plurality of said first communication links betweenthe respective nodes having a greatest available bandwidth.
 11. Acommunications network comprising a plurality of nodes, firstcommunication links for communicating user traffic between nodes, eachnode having at least one of said first communication links with each ofa plurality of respective neighbour nodes, and second communicationlinks for communicating control network traffic between at least some ofthe nodes, wherein each of the nodes is arranged for carrying out themethod of claim
 9. 12. A method as claimed in claim 6 wherein the stepof establishing a control network link for communicating control networktraffic directly with each of said respective neighbour nodes comprises,in the event of a fault adversely affecting a control network link usingone of said second communication links, maintaining the control networklink using available bandwidth of one of said first communication linksbetween the respective nodes.
 13. A communications network comprising aplurality of nodes, first communication links for communicating usertraffic between nodes, each node having at least one of said firstcommunication links with each of a plurality of respective neighbournodes, and second communication links for communicating control networktraffic between at least some of the nodes, wherein each of the nodes isarranged for carrying out the method of claim
 12. 14. A communicationsnetwork comprising a plurality of nodes, first communication links forcommunicating user traffic between nodes, each node having at least oneof said first communication links with each of a plurality of respectiveneighbour nodes, and second communication links for communicatingcontrol network traffic between at least some of the nodes, wherein eachof the nodes is arranged for carrying out the method of claim
 6. 15. Acommunications network comprising a plurality of nodes and a pluralityof communication links for communicating user traffic and controlnetwork traffic between the nodes, wherein each of the nodes is arrangedfor determining a plurality of neighbour nodes to each of which the nodehas a direct communication link for communicating user traffic, and isfurther arranged for automatically establishing and maintaining acontrol network link for communicating control network traffic directlywith each of said neighbour nodes, wherein the communication linkscomprise first links for communicating user traffic between nodes andsecond links for communicating control network traffic between nodes,and each node is arranged for automatically establishing the controlnetwork link using one of said second links where this is availablebetween the respective nodes, and otherwise establishing the controlnetwork link using available bandwidth of one of said first linksbetween the respective nodes.
 16. A communications network as claimed inclaim 15 wherein each node is arranged, for establishing the controlnetwork link using available bandwidth of one of said first linksbetween the respective nodes, to select one of a plurality of said firstlinks between the respective nodes having a greatest availablebandwidth.
 17. A communications network as claimed in claim 15 whereineach of said communication links has an identification of shared riskwith others of said communication links, and each node is arranged, forestablishing the control network link using available bandwidth of oneof said first links between the respective nodes, to exclude any of saidfirst links having a shared risk with any other control network link.18. A communications network as claimed in claim 17 wherein each node isarranged, for establishing the control network link using availablebandwidth of one of said first links between the respective nodes, toselect one of a plurality of said first links between the respectivenodes having a greatest available bandwidth.